Conventionally, a choke device for an engine to facilitate start-up by augmenting the concentration of fuel at the time of starting the engine is arranged, as shown in FIG. 2 and FIG. 3, by pivotally mounting, with shaft 7, a choke valve 5 for opening and closing a suction port 3 of a carburetor 2 provided on an air intake port 1 of the engine so as to be freely oscillatable in the direction perpendicular to an intake gas passage 9. By restricting the area of the sunction port 3 through an oscillatory operation of the choke valve 5, negative pressure created in the intake gas passage 9 is increased by the intake force of the engine. As a result, the amount of the fuel, jetting out of the main nozzle port 11 opened (its mouth) to the intake gas passage 9 is increased, so that the concentration of the fuel contained in sucked gas is also increased, thereby facilitating start-up of the engine.
With such a conventional structure, however, operation was cumbersome because it was necessary to appropriately operate the opening of the choke valve in addition to the starting operation for rotating the crankshaft, and moreover, it was necessary to quickly return and release the choke valve after the engine was started.
Furthermore, a large force was required for starting the engine due to lowering of the intake pressure caused by the closing of the choke valve.